1. Field of the Invention
This invention relates to solutions and processes for inhibiting corrosion. More specifically, this invention relates to solutions and processes for inhibiting corrosion of flow line surfaces that contain a mixture of hydrocarbons, water, and acid gases.
2. Description
Petroleum refiners have long sought a solution to corrosion of flow line surfaces that contain highly acidic mixtures of hydrocarbons, water, and acid gases such as carbon dioxide (CO.sub.2), hydrogen chloride (HCl), and hydrogen sulfide (H.sub.2 S). Refiners have had particular difficulty preventing corrosion caused by highly acidic mixtures, specifically those mixtures having a pH lower than 4-5, in refinery distillation tower overhead streams. As the steam in the overhead gas from a tower is condensed into liquid water at the surface of condensation equipment, some of the acid gases also condense and become dissolved in the liquid water or water phase. The resulting aqueous solution is highly acidic and thus corrodes the tower overhead piping, vessels, pumps, exchangers, and other equipment that will be familiar to one skilled in the art.
The conventional solution to the problem of inhibiting corrosion has been to add a neutralizing agent to the mixture of hydrocarbons, water, and acid gases to raise the pH. Such neutralizing agents have included ammonia and neutralizing amines, such as diethanolamine, methoxypropylamine, and morpholine. Some equipment operators have found that adding filming amines to the neutralizing amines further reduces corrosion of flow line surfaces.
Unfortunately, conventional filming amine corrosion inhibitors are ineffective in the low pH environment of petroleum refinery tower overhead streams. The conventional practice of adding neutralizing amines to elevate pH such that conventional filming amine corrosion inhibitors may be used has proved unsatisfactory. Above a pH of five (5), many naturally soluble species such as iron sulfide (FeS) and iron carbonate (FeCO.sub.3) form insoluble deposits that reduce the size of flow paths and eventually plug the equipment by totally blocking the flow path. Also, neutralizing amines may react to form corrosive salts and deposits that plug the flow lines of tower overhead piping and equipment. Therefore, the application of neutralizing amines in conjunction with conventional filming amines that do not inhibit corrosion in low pH environments has not satisfactorily solved the problem of inhibiting corrosion. The addition of neutralizing amines must be done judiciously, using as little neutralizing amine as possible to avoid possibly plugging equipment with deposits. In addition, reducing the amount of neutralizing amines used significantly reduces the overall cost of using the equipment to make a product, as neutralizing amines are relatively expensive.
Refiners have also had problems with conventional corrosion inhibitors that form oil/water emulsions. Water is typically separated from hydrocarbon streams, such as gasoline, to avoid contamination of the hydrocarbon product. Conventional corrosion inhibitors that form oil/water emulsions cause undesirable water entrainment in hydrocarbon streams. Thus, there is a need for a corrosion inhibitor that has little tendency to form oil/water emulsions.
Yet another problem with conventional corrosion inhibitors is inadequate distribution of the corrosion inhibitor throughout the corrosive stream. Conventional water-soluble corrosion inhibitors fail to adequately distribute themselves in a flow line that contains predominantly hydrocarbons in a hydrocarbon/water mixture. Poor distribution of conventional corrosion inhibitors causes inadequate corrosion protection of the piping and equipment surfaces because the chemical simply does not get to where it is needed. Thus, a need has long existed for a corrosion inhibitor that distributes itself evenly throughout a predominantly hydrocarbon stream and yet effectively prevents corrosion in the acidic aqueous phase. Solubility of the corrosion inhibitor in the hydrocarbon stream to which the inhibitor is added is a desirable characteristic. Because hydrocarbon solubility of a corrosion inhibitor enhances adequate distribution within the equipment to be protected, there is a need for a corrosion inhibitor that is hydrocarbon soluble yet still protects against corrosion in an aqueous phase.
Thus, there is a need for a corrosion inhibitor that (1) inhibits corrosion at low pH ranges, (2) is soluble in hydrocarbons such that the corrosion inhibitor is adequately distributed through the corrosive stream, (3) provides protection against corrosion in the aqueous phase of the fluid contained in the piping and equipment to be protected, and (4) does not cause an oil/water emulsion problem in the fluid contained in the flow lines to be protected, and more particularly, in the piping, condensers, pumps, vessels and other equipment in a distillation tower overhead system.